MERRILL is an open source 3D micromagnetics software package with a simple scripting user interface that requires little computational knowledge from the user in order to model complex imhomogeous domain structures in magnetic materials. MERRILL uses the finite-element/boundary-element numerical method best suited for modelling irregular grain geometries that are of interest to the rock and paleommagnetic community. It is able to simulate a range of magnetic characteristics of both single grains and small assemblies of interacting grains. The open source nature of the code encourages future development of the model by the scientific community. The software updates are tutorials will be published at www.rockmag.org Source code can be obtained from https://bitbucket.org/wynwilliams/merrill/src